Embodiments of the invention relate generally to electrical machines and, more particularly, to spoke rotor permanent magnet electrical machines in which rotor laminations are shifted along an axial length of the machine to reduce torque ripple, while still providing for use of a single extruded rotor shaft and block magnets.
Internal permanent magnet (IPM) machines such as IPM motors or generators have been widely used in a variety of applications, including aircraft, automobiles and industrial usage, and are currently the preferred machine employed in hybrid automotive applications. Therefore, a requirement for lightweight and high power density IPM machines has resulted in the design of higher speed motors and generators to maximize the power to weight ratios. Hence, the trend is increasing acceptance of IPM machines offering high machine speed, high power density, reduced mass and cost.
In a conventional IPM machine, multiple permanent magnets are embedded inside multiple laminations of a rotor. The mechanical stresses in the rotor are concentrated in multiple bridges and center posts. For higher speed applications, the thickness of the multiple bridges and center posts have to be increased for enhanced structural strength of the rotor and various other parts. The increased thickness leads to more magnet flux leakage into the multiple bridges and center posts, with such leakage significantly reducing the machine power density, so as to result in decreased efficiency of the machine.
The use of spoke rotors in IPM machines can decouple electromagnetic and mechanical requirements in an IPM machine by eliminating the need for bridges and center posts. This substantially increases the machine power density. Also, the spoke configuration has flux-concentration effects, which further increases the machine power density. Dovetailing rotor laminations onto the shaft allows higher speeds of the rotor outer radius, which further increases the machine power density. The rotor assembly and the various associated components can be configured to provide maximum power density and minimum eddy current losses. Furthermore, IPM machines having spoke rotors are advantageous in terms of low volume, mass and cost. Spoke rotor IPM machines thus allow for highly efficient permanent magnet machines.
However, spoke electric motors with distributed windings encounter the problem of high torque ripple. Torque ripple is undesirable in electric motors because it leads to transient losses and increasing the complexity of the control of the machine as well as mechanical issues in the drivetrain. Motors having stators with distributed windings usually encounter high torque ripple, while the use of spoke rotors with stators having distributed windings add additional harmonics, further increasing the torque ripple.
A standard approach to reducing torque ripple includes stator shifting by the slot pitch, and standard techniques introduce stator shifting by half a slot. Unfortunately, stator shifting or stepping along the axis can place stress on insulation material in the slots and reduces torque content in the machine. Also, stator shifting creates sharp edges which can cut into the slot insulation affecting the robustness and reliability of the motor.
Another method of reducing torque ripple known as rotor shifting eliminates problems with insulation, but rotor shifting is difficult because it complicates the construction of the rotor, leading to a more complicated shaft as well as manual insertion of magnets in shifted rotor laminations. The use of extruded shafts in rotor construction can reduce manufacturing complications, but may still require manual insertion of magnets at different axial locations or extrusion for different axial lengths.
Therefore, it is desirable to provide a spoke permanent magnet machine having a construction that reduces torque ripple, while eliminating the concerns associated with existing stator shifting and rotor shifting techniques and construction.